Process and apparatus for direct crystallization of polycondensates

ABSTRACT

The present invention relates to a process for continuous production of partly crystalline polycondensate pellet material, comprising the steps of forming a polycondensate melt into pellet material; separating the liquid cooling medium from the pellet material in a first treatment space, wherein the pellets after exit from the first treatment space exhibit a temperature T GR , and crystallizing the pellet material in a second treatment space, wherein in the second treatment space fluidized bed conditions exist, and in the second treatment space the pellets are heated by supply of energy from the exterior by means of a process gas.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 14/032,357 filed Sep. 20, 2013, which claims priority fromEuropean patent application serial no. 12186020.9 filed Sep. 26, 2012.

The present invention relates to a process and apparatus for directcrystallization of polycondensates, especially polyesters such aspolyethylene terephthalate (PET).

The production of certain high molecular weight polymers, especiallypolycondensates such as polyesters, is frequently not possible via meltpolycondensation, since the excessive thermal stress associatedtherewith is responsible for product degradation and an unacceptablecontamination with by-products. Therefore, prepolymers having acomparatively low molecular weight are produced via meltpolycondensation. The prepolymers are subsequently raised to the desiredmolecular weight in a solid-state polymerization (SSP). This procedureis known from the prior art (e.g., Scheirs/Long (eds.), ModernPolyesters, Wiley 2003, chapter 4, pp. 143 ff.).

The prepolymer obtained by melt polycondensation is processed into apellet material for the SSP reaction. To avoid agglomeration of thepellets under the conditions of the SSP reaction due to stickiness, thepellet material is partly crystallized prior to the SSP reaction. Thisis also known from the prior art (e.g., Scheirs/Long (eds.), ModernPolyesters, Wiley 2003, chapter 4, pp. 158-164).

If the high-molecular polycondensate can be manufactured by meltpolymerization, a subsequent partial crystallization may still benecessary, in order to enable further processing steps, such asdevolatilization.

Usually, the polycondensate or polycondensateprepolymer is cooled downafter it has been formed into pellets and reheated for thecrystallization. However, processes are also known from the prior artwherein the asformed pellet material is fed in the hot state to thecrystallization stage without cooling down in-between. Processes of thistype are known as direct-crystallization processes. DE 103 49 016 and DE10 2004 015 515, for example, describe so-called latent heatcrystallization processes wherein crystallization is effected usingsolely the intrinsic heat of the pellets. But these processes have thedisadvantage that they are unable to meet the required flexiblyadjustable and homogeneous quality of the output with regard totemperature and crystallinity. The frequent formation of agglomerates inthe upstream-end region of the crystallization zone is a furtherdisadvantage because they do not always break up again completely.

Better results are obtainable when the crystallization of the still hotpellets is effected by additional heating with a hot process gas. Thisis generally suggested, for example, in U.S. Pat. No. 3,544,525, butthat document does not disclose process details. As discussed below,said method is associated with several drawbacks.

WO 2008/071023 describes a process wherein still hot pellet material isseparated from the cooling medium in a pellet dryer and subsequentlytransferred into a crystallizer where it is treated under definedconditions. Both the pellet dryer and the crystallizer are operated hereunder an air atmosphere. Evaporated cooling medium can be removed bymeans of air. The flow velocity of the process gas in the crystallizerhas to be above the loosening point of the polycondensate, and thepolycondensate pellets have to exhibit a narrow spectrum of residencetime in the crystallizer, in order to obtain homogeneously crystallizedpolycondensate pellets. In order to obtain a narrow spectrum ofresidence time, at least in parts of the crystallizer a strongturbulence has to be prevented. These specific process conditions arelimiting and thus disadvantageous.

However, crystallization is not always possible under an air atmosphere,since oxidative degradation can occur at the high temperatures ofcrystallization. With certain materials/quality requirements, therefore,it is necessary to crystallize under an inert gas atmosphere.

U.S. Pat. No. 3,544,525 mentioned above describes a process whereinpolymer melt is formed into a prepolymer pellet material via anunderwater pelletization. The pellet material is subsequently dried in adewatering unit (pellet dryer) and transferred into a crystallizationdevice where it is crystallized with hot inert gas.

This process has various disadvantages. In addition to crystallizationcontrol and homogeneity being unsatisfactory, the design of the pelletdryer as a closed system causes an overpressure to develop therein dueto the evaporating cooling liquid (water). As a consequence, the wateris forced out of the pellet dryer and into the crystallizer and carriedinto the adjoining SSP reactor. Although the prepolymer is described asrelatively dry in U.S. Pat. No. 3,544,525 after passing through thepellet dryer, the SSP reactor nonetheless has a high moisture contentwhich is absorbed by the process gas flowing through the SSP reactor.Before returning into the SSP reactor, the process gas has to be freedof water in additional units (a condenser and a dryer), which isinconvenient. Alternatively, the moist process gas can be disposed ofand replaced with fresh dry process gas. This variant is costly andhence unsatisfactory. In addition, the high moisture content of the SSPreactor causes the pellets to cool down, since the water adheres to thepellets and has to be evaporated. More process gas is accordingly neededin the SSP reactor to achieve the desired reaction temperature, and thisis economically disadvantageous.

The problem addressed by the present invention was that of providing aprocess and apparatus for direct crystallization of polymer which do nothave the disadvantages of the prior art.

It was found according to the present invention that, surprisingly, thedisadvantages of the prior art are overcome when crystallization in thecrystallizer is conducted with specific supply of energy from theexterior.

The present invention relates to a process for continuous production ofpartly crystalline polycondensate pellet material, comprising the stepsof

-   -   a) forming a polycondensate melt into pellet material by adding        a liquid cooling medium, which has a temperature below the glass        transition temperature (Tg) of the polycondensate, and cooling        to an average pellet temperature within the range of temperature        of crystallization of the polycondensate, wherein cooling takes        place before or during or after forming to pellets;    -   b) separating the liquid cooling medium from the pellet material        in a first treatment space, wherein the pellets after exit from        the first treatment space exhibit a temperature T_(GR),    -   c) crystallizing the pellet material in a second treatment        space,

wherein in the second treatment space fluidized bed conditions exist,and in the second treatment space the pellets are heated by supply ofenergy from the exterior by means of a process gas, wherein the processgas has a temperature T_(Gas), which is higher than the sum of thepellet temperature T_(GR) and the temperature increase T_(KR) whichoccurs due to heat of crystallization released in the second treatmentspace, i.e. T_(Gas)>(T_(GR)+T_(KR)), and wherein the pellets at the exitfrom the second treatment space have an average temperature T_(PH),which is 5 to 70° C. higher than the sum of the temperature of thepellets T_(GR) and the temperature increase T_(KR) which occurs due toheat of crystallization released in the second treatment space, i.e.(T_(GR)+T_(KR)+70° C.)≥T_(PH)≥(T_(GR)+T_(KR)+5° C.)

The present invention furthermore relates to a device for performing theabove described method, comprising

-   -   a unit for forming pellets with a line for supply of a cooling        medium and a line for discharge of a mixture of pellets/cooling        medium,    -   a drying unit located downstream of the unit for forming        pellets, providing a first treatment space,    -   and a crystallizer located downstream of the drying unit,        providing a second treatment space and being provided with each        an inlet and outlet for the pellets and a gas, so that in the        second treatment space the gas can be led through the pellets        under fluidized bed conditions,

wherein the crystallizer is provided at the outlet for the pellets witha shut-off device, preferably a rotary air lock unit with a housing anda rotor movably arranged therein and having a gap between the rotor andthe housing which is larger than the average diameter of the pellets.

The present invention is provided for treating crystallizablepolycondensates. Suitable polycondensates include crystallizablethermoplastic polycondensates, for example polyamides, polycarbonates,and polyesters including polyhydroxy alkanoates, polylactides or theircopolymers, which are obtained via a polycondensation reaction byeliminating a low molecular weight reaction product. Thepolycondensation in question can take place directly between themonomers or via an intermediate stage which is subsequently converted bytransesterification, in which case the transesterification can in turntake place by eliminating a low molecular weight reaction product or viaring-opening polymerization. Essentially, the polycondensate thusobtained is linear, wherein a low number of branching can be generated.

Polycondensates of a certain type of polymer are each made from the samemain monomers. A limited amount of further monomers, so-calledco-monomers, can be used therein.

Polyamides are polymers usually obtained via polycondensation from adiamine component with the general structure H₂N—R1-NH₂ and adicarboxylic acid component with the general structure HOOC—R2-COOH,wherein R1 and R2 typically are optionally substituted, linear orbranched aliphatic hydrocarbons with 1 to 15 carbon atoms, aromatic orheteroaromatic hydrocarbons with 1 to 3 aromatic rings, cyclichydrocarbons with 4 to 10 carbon atoms or heterocyclic hydrocarbons with1 to 3 oxygen or nitrogen atoms and 3 to 10 carbon atoms.

Instead of the dicarboxylic acid, also its corresponding dicarboxylicacid halogenide, preferably dicarboxylic acid chloride, can be used.

Examples of such polyamides are PA6,6, which is made from hexamethylenediamine and adipic acid, or PA-mXD6, which is made from m-xylylenediamine and adipic acid.

Polyamides are also polymers with repeating amide groups with thegeneral structure H—[N(H)—R—CO]x-OH, wherein R typically is anoptionally substituted, linear or branched aliphatic hydrocarbon with 1to 15 carbon atoms, aromatic or heteroaromatic hydrocarbon with 1 to 3aromatic rings, cyclic hydrocarbon with 4 to 10 carbon atoms orheterocyclic hydrocarbon with 1 to 3 oxygen or nitrogen atoms and 3 to10 carbon atoms.

Polyamides are also polymers which are made by ring-openingpolymerization from heterocyclic monomers with at least one amide group,such as for example polycaprolactame (PA6), which is manufactured fromcaprolactame.

Polyesters are polymers typically obtained via polycondensation from adiol component with the general structure HO—R1-OH and a dicarboxylicacid component with the general structure HOOC—R2-COOH, wherein R1 andR2 typically are optionally substituted, linear or branched aliphatichydrocarbons with 1 to 15 carbon atoms, aromatic or heteroaromatichydrocarbons with 1 to 3 aromatic rings, cyclic hydrocarbons with 4 to10 carbon atoms or heterocyclic hydrocarbons with 1 to 3 oxygen ornitrogen atoms and 3 to 10 carbon atoms.

Usually linear or cyclic diol components and aromatic or heterocyclicdicarboxylic acid components are used. The ester, usually the dimethylester of the dicarboxylic acid can also be used instead of thedicarboxylic acid itself.

Typical examples of polyesters are polyethylene terephthalate (PET),polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT),polyethylene furanoate (PEF), polytrimethylene furanoate (PTF),polybutylene succinate (PBS and polyethylene naphthalate (PEN), whichare each used either as homopolymer or as copolymers.

An example is polyethylene terephthalate, which is obtained from itsmonomers, a diol component and a dicarboxylic acid component, the maindiol component being ethylene glycol (1,2-ethanediol) and the maindicarboxylic acid component being terephthalic acid. Possible comonomersinclude further linear, cyclic or aromatic diol and dicarboxylic acidcomponent compounds. Typical comonomers are diethylene glycol (DEG),isophthalic acid (IPA) or 1,4-bishydroxymethylcyclohexane (CHDM).

A further example is polyethylene furanoate, which is obtained from itsmonomers, a diol component and a dicarboxylic acid component, the maindiol component being ethylene glycol (1,2-ethanediol) and the maindicarboxylic acid component being 2,5-furane dicarboxylic acid. Possiblecomonomers include further linear, cyclic or aromatic diol anddicarboxylic acid component compounds. Typical comonomers are diethyleneglycol (DEG) or trimethylene glycol.

Polyesters are also polymers with repeating ester groups with thegeneral structure H—[O—R—CO]_(x)—OH, wherein R typically is anoptionally substituted, linear or branched aliphatic hydrocarbon with 1to 15 carbon atoms, aromatic or heteroaromatic hydrocarbon with 1 to 3aromatic rings, cyclic hydrocarbon with 4 to 10 carbon atoms orheterocyclic hydrocarbon with 1 to 3 oxygen or nitrogen atoms and 3 to10 carbon atoms.

An example are polyhydroxy alkanoates with the general structureH—[O—C(R)H—(CH₂)_(n)—CO]_(x)—OH, wherein R is usually hydrogen, or alinear or branched aliphatic hydrocarbon with 1 to 15 carbon atoms andn=1 to 10. Examples are poly-4-hydroxybutyrate andpoly-3-hydroxyvalerate.

A further example are polylactides with the general structureH—[O—C(R)H—CO]_(x)—OH, wherein R is typically a methyl group or analiphatic hydrocarbon with 1 to 15 carbon atoms.

A further example is the polyglycolic acid with the general structureH—[O—CH₂—CO]_(x)—OH].

Polyesters are also polymers which can be made by ring openingpolymerization from heterocyclic monomers with an ester group, such asfor example polycaprolactone from caprolactone, or by ring openingpolymerization from heterocyclic monomers with at least two estergroups, such as polylactide from lactide.

The most common polylactide is polylactic acid with the structureH—[O—C(CH₃)H—CO]_(x)—OH. Due to the chirality of the lactic acid, thereexist different forms of polylactic acid. Homopolymers arepoly-L-Lactide (PLLA), which is usually made from L,L-lactide, andPoly-D-lactide (PDLA), which is usually made from D,D-lactide.

Copolymers such as poly-(L-lactide-co-D,L-lactide) contain littleamounts of lactide units with chirality that it different from that ofthe main monomer.

Polyesters can also be manufactured by biosynthesis usingmicroorganisms, or in plant cells, where they are obtained bydestruction of the cell.

Suitable polycondensates can be crystallizable homopolymers. Despite thedesignation as homopolymer, during the manufacturing process thereof asmall amount of comonomers may be formed. Thus, during manufacture ofpolyethylene terephthalate, it is known that diethylene glycol is formedfrom ethylene glycol. However, many suitable polycondensates arecrystallizable copolymers which contain a certain amount of comonomer.The comonomers can be introduced into the manufacturing process of thepolycondensate as portion of the monomers, or they are formed as part ofthe manufacturing process, typically resulting in an arbitrarydistribution. The comonomers may also be introduced in the form ofblocks, manufactured from different monomers, resulting in so-calledblock copolymers.

Typical amounts of one or more comonomers are from 1% to about 30%. Dueto the fact that in many cases an excessive amount of comonomercompletely prevents crystallization, the maximum amount of comonomer maybe limited to below 20%, preferably below 10%.

Suitable polycondensates may be mixtures of polymers which may containan arbitrary number and amount of different types of polymers. A smallamount of a polymer may act as nucleating agent in a polycondensate andthus increase its rate of crystallization. Specific mixtures ofpolycondensates can form mutually interacting crystal structures withcrystallization characteristics which differ from that of the individualcomponents.

A respective example is a mixture of PDLA and PLLA, which forms astereocomplex crystal structure with increased crystallinity.

After polymerization, each polycondensate chain has chain-terminatinggroups with typically a functionality of at least one of its monomers.As an example, a polyester chain may have one or more hydroxyl and/orcarboxyl end groups. A polyamide chain may have one or more hydroxyland/or amino end groups. Such terminal groups may be modified by aso-called end-capping reagent, or they can be modified due to adegradation reaction. Although this was not specifically mentioned withrespect to the above general structures, suitable polymers may have suchmodified terminal groups.

The polycondensate can be virgin material or a recyclate. Recyclates arereprocessed polymers from the production and processing operations(post-industrial) or polymers collected and reprocessed after havingbeen used by consumers (post-consumer).

Additives may be incorporated in the polycondensate. Suitable additivesinclude, for example, catalysts, dyes and pigments, UV blockers,processing aids, stabilizers, impact modifiers, blowing agents of thechemical and physical kinds, fillers, nucleating agents, flameretardants, plasticizers, particles to improve the barrier or mechanicalproperties, reinforcing articles, such as balls or fibres, and alsoreactive substances, for example oxygen absorbers, acetaldehydeabsorbers or molecular weight enhancers.

The unit which is used in the present invention for forming apolycondensate pellet material is fed with a polycondensate melt asstarting material. The polymer melt is produced using reactors ordevices known in the prior art (e.g., Scheirs/Long (eds.), ModernPolyesters, Wiley 2003, especially pp. 31-104). In-principlepossibilities include polymerization reactors in which thepolycondensates are produced in the liquid phase, for example stirredtanks, cage reactors or disc reactors, or else equipment wherepreviously produced polycondensates are melted, for example extruders orkneaders. Polycondensate melt production can be continuous or batchwise.However, continuous processes are preferable for further processing.

Individual strands of polycondensate are formed from the polycondensatemelt in an exit apparatus, especially a die or die-plate. To producepellets (i.e. particles of defined shape and size) from the strands ofpolycondensate, the pelletization techniques known in the prior art,such as strand pelletization, water-ring pelletization, underwaterpelletization or hot face pelletization, can be used. In pelletization,the strands of polycondensate which exit from the melt channels aresolidified and separated into a multiplicity of individual pelletsbefore or after solidification. Separation is effected, for example, byautogenous droplet formation, by the use of a liquid shearing medium orby mechanical severing, especially cutting. While droplet formation,whether autogenous or forced by a shearing medium, takes place at thedie exit, cutting can not only take place directly at the die exit, butalso only after passage through a treatment sector.

The polycondensate melt is solidified by cooling with at least oneliquid cooling medium or a mixture of different liquid cooling media.Especially suitable cooling media are liquids which have a high specificcapacity of heat, preferably higher than 2 KJ/(kg·K), and a sufficientlyhigh boiling point, preferably higher than 90° C., and which essentiallydo not attack or modify the polycondensate and do not leave behind anytoxic residues in the polycondensate. According to a preferredembodiment of the invention, a single liquid cooling medium is used.Water or ethylene glycol or mixtures thereof are preferred. Anespecially preferred cooling medium is water.

The polycondensate, especially in the form of polycondensate strands orin the form of droplets, can, for example before entry into the liquidcooling medium, flow through a sector which contains a process gas,especially air or water mist. Notwithstanding the use of the term“water” in the designation of pelletizing means, other liquid media canalso be used. Cooling can take place according to the present inventionbefore, during or after the forming of the material into pellets.

According to a first embodiment of the invention, when entering the unitfor forming the pellets the liquid cooling medium has a temperaturewhich is below the glass transition temperature (also called glasstransition point, abbreviated with Tg) of the polycondensate to beshaped. In the case of polyesters, the pellets begin to agglomerateabove the glass transition temperature of the material, i.e. theparticles stick together forming agglomerates. For example, the glasstransition temperature of polyethylene terephthalate is about 75° C. to82° C. (depending on the comonomer content and the kind of addedcomonomers).

The glass transition temperature of a polycondensate may be determinedwith a DSC (digital scanning calorimetry) measurement. DSC is aconventional method commonly known to a skilled person. Devices forperforming DSC measurements are also commonly known and commerciallyavailable. For example, the device Mettler DSC81 may be mentioned. Formeasuring the glass transition temperature of a polycondensate, such asa polyester, for example 5-25 mg of a respective polymer sample can beheated in a conventional Mettler DSC81 measurement device with a rate of10° C./min from 25° C. to 290° C. The sample is kept at 290° C. for 1min, then rapidly cooled down to room temperature and heated for asecond time with a rate of 10° C./min from 25° C. to 290° C. The glasstransition temperature is determined from the second run. As Tg, thepoint of inflection of the respective peak in the DSC is taken.

According to a preferred embodiment of the invention, the liquid coolingmedium, preferably water, depending on the Tg of the polycondensate tobe shaped correspondingly has a temperature of less than 85° C.,particularly preferred of between 50° C. and 80° C. and especiallypreferred of between 60° C. and 75° C.

According to a further embodiment of the present invention, the liquidcooling medium when entering the unit for forming the pellets has atemperature which is more than 50° C., but lies at least 10° C. belowits boiling point. In the case of water, according to this embodimentthe temperature of the cooling medium at normal pressure is thus 50° C.to 90° C. The boiling point is known to be dependent on pressure; withdecreasing external pressure the boiling point decreases, and viceversa.

According to a preferred embodiment of the invention, the liquid coolingmedium is guided through a circular system, for economic reasons. Duringthe pelletization mentioned above, however, the amount of liquid coolingmedium (preferably water) decreases, since a part thereof evaporates.This applies in particular to embodiments where pelletization is carriedout in a liquid cooling medium with increased temperature. The portionof liquid cooling medium escaping the circular system has to beregularly replaced, in order to provide the amount of cooling medium inthe system which is required for pelletization. The amount of freshcooling medium thus typically is 5 to 200 litres per ton of manufacturedpolycondensate pellets. Optionally, evaporated cooling medium can becondensed and led back into the circular system, resulting in a decreaseof fresh cooling medium.

In U.S. Pat. No. 7,446,162, normal tap water (“city water”) is suggestedas replacement for escaped liquid cooling medium, optionally togetherwith additives for the control of water hardness or for complying withdirectives for corrosion. However, no further respective details areprovided.

It was found, however, that this procedure is disadvantageous. If aliquid cooling medium comprising lager amounts of non-volatilecomponents is added to the circular system, over a longer period of timean uncontrolled enrichment of those non-volatile components in thecircular system takes place. This can lead to undesirable drawbacks suchas deposition of limescale in the circular system, which can onlyprevented to a certain degree by the addition of additives.

Moreover, during pelletization of polycondensates side products from theprocess of preparation of the polycondensate (such as monomers,oligomers or cleavage products formed therefrom) enter into the liquidcooling medium. Particularly critical is here the entering of acidiccomponents (such as the acid component of polyesters or cleavageproducts from polyester monomers) into the liquid cooling medium, sinceherewith the pH value of the cooling medium is lowered. As aconsequence, due to corrosion the life time of metallic components ofthe unit, especially the die plates or the cutting knifes, is lowered.

According to an embodiment of the present invention, the above problemsare overcome by adjusting and keeping the pH value of the liquid coolingmedium present in the circular system to a value of more than 4,preferably to a value in the range from 4 to 7. This can be achieved,for example, by adding to the liquid cooling medium (preferably water)which is given to the circular system as replacement for escaped coolingmedium, a basic medium or a pH buffer medium. Alternatively, the basicmedium or the pH buffer medium can also be given directly into thecircular system.

According to a preferred embodiment of the present invention, to thecircular system there is added an amount of 0.0005-0.05 mol, preferably0.0015-0.05 mol, especially 0.005-0.05 mol, of basic medium or pH buffermedium per ton of polycondensate to be pelletized. Therewith, it isensured that under conditions of pelletization of the invention the pHvalue of the liquid cooling medium in the circular system is kept above4 and preferably in the range from 4 to 7.

As basic medium, organic or inorganic components may be used, but alsoampholytes such as glycine may be used.

As basic pH buffer medium, organic or inorganic buffer systems may beused which stabilize the pH value at a level of more than 4, preferablyin the range from 5-9. Examples are phosphate, hydrogenphosphate,carbonate or hydrogencarbonate buffers, which have to be soluble in theliquid cooling medium.

According to a preferred embodiment of the invention, a carbonatebuffer, such as sodium carbonate, or a hydrogencarbonate buffer, such assodium hydrogencarbonate or calcium hydrogencarbonate, are used as pHbuffer medium.

The determination of the pH value of a liquid medium is commonly knownand does not have to be explained here in detail.

According to a particularly preferred embodiment of the invention, asreplacement for escaped cooling medium the circular system is provided,as liquid cooling medium (preferably water), with fresh water having acarbonate hardness of 0.2 to 5° fH, preferably 0.5 to 5° fH. Carbonatehardness is the amount of earth alkaline ions, mainly magnesium (Mg²⁺)and calcium (Ca²⁺), and traces of barium (Ba²⁺) and strontium (Sr²⁺),which together with carbonate ions (CO₃ ²⁻) and hydrogencarbonate ions(HCO₃ ⁻) are dissolved in the water. Carbonate hardness thus onlycomprises the earth alkaline ions which are present as dissolvedcarbonates or hydrogencarbonates. ° fH here is the hardness in Frenchdegrees, wherein 1° fH is 0.1 mmol of earth alkaline ions per liter.

The adjustment of water hardness is typically made by partial softeningof untreated fresh water, wherein usually completely softened water ismixed with untreated fresh water.

When adjusting the water hardness, addition of chloride ions should beavoided. If necessary, a reduction of chloride ions to below 10 mg/l, inparticular 2 mg/l, should be carried out.

According to a further preferred embodiment of the invention,furthermore a part of the liquid cooling medium is removed from thecircular system, wherein this part corresponds to at least 10%, but notmore than 100%, preferably 20 to 80%, of the amount of fresh coolingmedium which is added to the circular system. Therewith, in particularan enrichment of chloride ions can be prevented, wherein a chloridecontent of 20 mg/l should not be exceeded.

According to a further preferred embodiment of the invention, thecooling medium in the circular system is adjusted in such a way that itcontains, before being added to the polycondensate, oxygen in an amountof less than 7 mg/l, preferably less than 5 mg/l. This can be achieved,for example, by degassing the liquid cooling medium at elevatedtemperature, in particular at a temperature of above 60° C. Degassingcan be carried out, for example, with a vacuum pump or a suctionventilator. Optionally, an open aeration opening allowing the escape ofreleased oxygen gas is also sufficient. This is supported by a partialpressure of the oxygen as small as possible in the gas phase at theboundary of the cooling liquid, which can be achieved, for example, byaddition of nitrogen or by evaporating cooling medium. Therewith, aconstant quality of the liquid cooling medium is ensured, and a negativeinfluence on the polycondensate is prevented.

The present invention is thus furthermore related to a process forcontinuous production of partly crystalline polycondensate pelletmaterial, comprising the steps of

-   -   a) forming a polycondensate melt into pellet material by adding        a liquid cooling medium, which has a temperature above 50° C.,        but at least 10° C. below its pressure-dependent boiling point,        and cooling to an average pellet temperature within the range of        temperature of crystallization of the polycondensate, wherein        cooling takes place before or during or after forming to        pellets;    -   b) separating the liquid cooling medium from the pellet material        in a first treatment space,    -   c) crystallizing the pellet material in a second treatment        space,

wherein the liquid cooling medium is guided in a circular system, andwherein the liquid cooling medium before addition to step a) has a pHValue of more than 4, preferably 4 to 7.

Partly crystalline (or partially crystalline) polycondensate materialhere designates polycondensate pellets which comprise both amorphouszones and zones with crystalline structure.

The pellets obtained in this way shall preferably have a defined pelletshape, for example cylinder-shaped, sphere-shaped, droplet-shaped,sphere-like or a designed shape as proposed in EP 0 541 674 for example.Average pellet size shall be between 0.1 mm and 10 mm, preferablybetween 0.5 mm and 3 mm and especially preferred between 0.85 mm and 2.5mm.

The average pellet size is the statistical mean value of the averagepellet diameter, which is derived from the ratio of pellet height,pellet length and pellet width (which can be measured by known methods).The pellet size distribution should be kept within a narrow spectrum.According to a preferred embodiment, the standard deviation of theweights of 100 measured pellets is between 2 and 20%.

According to the present invention, the pellet material obtained issubjected to direct crystallization, i.e. the pellet material is notcooled down so severely (to room temperature, for example) that for asubsequent crystallization it would have to be heated with high energyrequirements. According to the present invention, the pellet material iscooled down to an average pellet temperature which is within thecrystallization temperature range of the polycondensate. This isachieved by choosing the temperature of the cooling medium as describedabove and by optionally choosing a correspondingly short residence timefor the pellet material in the cooling medium. According to a preferredembodiment of the invention, separation of the cooling medium from thepellets takes place within 0.1 to 5 s, especially preferred within 0.3to 3 s after addition of the cooling medium. Concurrently with thecooling, the polycondensate pellets can be conveyed into a furtherprocessing step.

The average pellet temperature here designates the mean value of thetemperatures of the individual pellets, wherein each pellet has a meantemperature which is established after a balance of the temperatureprofile in the pellets without heat exchange with the exterior has beenreached.

The suitable range of temperature for the crystallization becomesapparent on plotting the crystallization half time (t½) as a function ofthe temperature. Said range is upwardly and downwardly limited by thetemperature at which the crystallization half time is approximatelyequal to 10 times the minimum crystallization half time (t½ min). Sincevery short crystallization half times (t½) are very difficult todetermine, t½ min=1 minute is used as minimum value. In the case ofpolyethylene terephthalate, for example, the suitable crystallizationtemperature range is between 110 and 220° C.; in the case ofpolyethylene furanoate the suitable crystallization temperature range isbetween 110 and 190° C., and in the case of poly-L-lactide the suitablecrystallization temperature range is between 80 and 140° C.

The crystallization half time (t½) is determined by isothermalcrystallization in the DSC (differential scanning calorimetry). Thecrystallization half time corresponds to the time which, at a giventemperature, is needed to attain 50% of the attainable crystallinity.The minimum crystallization half time (t½ min) is the shortestcrystallization half time which can be attained in the crystallizationtemperature range.

According to a preferred embodiment of the invention, in the case ofpolyethylene terephthalate (PET) as polymer for treatment, the pelletmaterial is cooled down to a temperature in the range from 110 to 180°C., preferably from 115 to 160° C. and more preferably from 120° C. to150° C.

After cooling, the cooling medium is separated from the pellets.Optionally, the pellets are subjected to a further treatment(conditioning) in a liquid medium, for which directly the cooling mediumor some other liquid can be used.

The pellets can be separated from a liquid cooling medium usingseparating apparatuses known in the prior art. They may merely bepassive separating apparatuses, for example grids or grills, throughwhich the cooling medium can pass but not the pellet material. Activeseparating apparatuses are usually used for at least part of theseparation, the separation being effected for example due to a gasthrough-flow, a centrifugal force or an impact. Apparatuses of this typeare known for example as suction apparatuses, impact dryers orcentrifugal dryers. A centrifugal dryer is preferred.

According to a preferred embodiment of the present invention, thepellets are separated from the cooling medium in a two-step process,i.e. according to the present invention the step of separating theliquid cooling medium from the pellets may comprise the above describedseparation and subsequent drying of the pellets. After a firstseparation with the above described separating apparatuses, according tothis preferred embodiment during the step of separating from the coolingmedium the pellets are furthermore subjected to a drying step in orderthat the cooling medium may be completely separated from the pelletmaterial. For this, the pellet material is transferred from anabove-described unit for forming the pellet material via a connectionline into a unit for separating the cooling medium and into a unit(drying unit) for drying the pellet material. The transfer of the pelletmaterial into the drying unit is effected at such a rate of speed thatthe pellet material does not cool down below its crystallizationtemperature range. The flow velocity of the pellet material in theconnection line can be increased by importing air or some other suitablegas into the connection line.

Units for drying a pellet material (drying units) are known from theprior art. The preference of the present invention is for using areactor in which a first treatment space is provided. Inside thereactor, the cooling medium (generally water) is separated off and, inthe first treatment space, the pellet material separated off is dried byevaporating the cooling medium. To evaporate the cooling medium, atemperature in the range from 100 to 200° C., preferably from 110° C. to160° C. and more preferably from 120 to 150° C. is applied to the firsttreatment space. The residence time of the pellet material in the firsttreatment space is preferably in the range from one tenth of a second upto 10 seconds.

According to a preferred embodiment of the invention, after exit fromthe first treatment space the pellets exhibit a temperature T_(GR) inthe range from 100-180° C., preferably 120° C. to 160° C.

The pellet dryer of the present invention has at least one feed-inopening for importing the pellet material into the unit. The feed-inopening can be for example an opening in the housing, or the point ofexit from a pipe leading into the housing. The pellet dryer of thepresent invention further has at least one discharge opening forexporting the pellet material out of the unit. The discharge opening canbe for example an opening in the housing or the point of entry into apipe leading out of the housing. Furthermore, the drying unit of theinvention comprises at least one discharge opening for removing theliquid cooling medium from the unit.

A gas phase is present in the first treatment space and absorbs theevaporating cooling medium. According to the present invention, the gasis preferably air. But other gas mixtures having a lower oxygen contentthan air can also be used. An inert gas need not be used. In exemplaryembodiments, the oxygen content of the first treatment space can be morethan 1% by weight, preferably more than 5% by weight and more preferablymore than 10% by weight. To avoid oxidative degradation of the material,however, the oxygen content of the first treatment space should notexceed the oxygen content of air (around 21%).

The drying unit of the present invention preferably is not a closedunit. The drying unit of the present invention preferably has at leastone exit opening for exporting gas, preferably air. In a preferredembodiment of the present invention, the exit opening of the drying unitempties into a gas exportation line which accommodates a ventilator forair circulation through the drying unit.

The drying unit of the present invention may further have at least oneentry opening for importing gas, preferably air. The entry opening inthis case is disposed at the opposite end of the first treatment spacefrom the exit opening in order that complete flow of gas through thefirst treatment space may be ensured. However, it is also possible forthe entry opening for importing gas to be disposed in the connectingunit immediately downstream of the drying unit and not in the dryingunit itself.

In a preferred embodiment of the present invention, the gas passes tothe entry opening via an aspirating filter. A ventilator for aircirculation through the drying unit can be disposed in the gas lineleading to the entry opening. This ventilator can be provided inaddition to or in place of the ventilator in the gas exportation line.In a preferred embodiment of the present invention, therefore, aventilator is only provided in the gas importation line.

In a further embodiment of the present invention, the line leading tothe entry opening and the line leading away from the exit opening cancommunicate with each other to form a closed-loop circuit. In thisembodiment, however, the gas first has to pass through a condenserbefore reentry into the first treatment space in order that theevaporated cooling medium in the gas may be separated off.

The apparatus of the present invention preferably has a cooling-mediumcircuit. The cooling medium is fed from a stock reservoir vessel (tank),preferably via a circulating pump and optionally a heat exchanger (forselectively heating or cooling the cooling medium), into the formingunit (pelletization apparatus). The cooling medium separated off in thedrying unit can be led back into the stock reservoir vessel via apipework line.

Given a constant ratio of air importation and exportation, a pressureincrease will occur in the first treatment space during the dryingoperation, since the liquid cooling medium transitions into the gaseousstate and the gas phase in the first treatment space increases.

Separation of the polycondensate pellets from the liquid cooling mediumis followed by a direct transfer into the subsequent crystallizer, whichcomprises a second treatment space. This, according to the presentinvention, is accomplished by means of a connection line which isdisposed between the drying unit and the crystallizer and connects thedischarge opening of the drying unit to the inlet opening of theafter-connected crystallizer. The connection line is preferablyconfigured such that the material to be treated can pass unimpeded fromthe pre-connected unit to the downstream unit. Preferably, there areaccordingly no shut-off devices in the connection line. This is intendedto rule out the risk of the hot, tacky particles of the polymer adheringin the connection line. The connection line can have a gas inlet.

Constant relative movement between the individual pellets should beensured in the connection line in order that their agglomeration due tostickiness may be prevented. The relative movement is achievable, forexample, via a high flow velocity in the connection line (greater than0.3 m/min, especially greater than 1 m/min), via an encircling flow witha conveying gas, via mechanical movement, for example using a stirrer orconveying screw, or by generating a vibration or oscillation.

Optionally, between drying unit and crystallizer a sieve is provided,through which individual pellets with specified size can protrudeunobstructed, but which holds back pellet agglomerates and too largepellets.

In order that, in the event of a system upset of the crystallizer,inconvenient and costly exportation of material may be avoided, theconnection line of the present invention can, as described in WO2008/071278, communicate with an intermediate store into which thematerial is passed by a controlling apparatus in the event of a systemupset and is stored therein under conditions (especially pellettemperature below pellet glass transition point) where agglomeration ofthe pellets does not occur. The corresponding content of WO 2008/071278is hereby expressly incorporated herein by reference.

As the pellet material to be treated enters the crystallizer it is ingeneral essentially amorphous, i.e. its crystallinity is less than 10%,preferably less than 5%. According to the present invention, pelletmaterial made of polyethylene terephthalate (PET) preferably has an IVvalue of 0.4 to 0.8 dl/g, especially 0.5 to 0.65 dl/g, before entry intothe crystallizer. The IV value indicates the intrinsic viscosity of apolymer and is a measure of its molecular weight. The IV value and itsdetermination are known from the prior art. The intrinsic viscosity (IV)is determined, according to the present invention, according to thefollowing method: For determining the solution viscosity, a mixture ofphenol/dichlorobenzene (50:50 wt.-%) is used as solvent. The polyestersample is dissolved during 10 minutes at 130° C. with a concentration of0.5% (0.5 g/dl). Measurement of the relative viscosity (R.V.) is carriedout at 25° C. with an Ubbelohde viscosimeter (according to DIN norm no.53728, part 3 dated January 1985). The relative viscosity is thequotient of the viscosity of the solution and the viscosity of the puresolvent, which is comparable to the ratio of the corresponding capillaryflow velocities. According to the Huggins equation, the value of theintrinsic viscosity is calculated from the measured relative viscosity;

${I.V.} = \frac{\sqrt{1 + {4{K_{H}\left( {R.V.{- 1}} \right)}}} - 1}{2^{\star}c^{\star}K_{H}}$

With the above conditions of measurement (polymer concentration c=0.5g/dl and Huggins constant (K_(H)=0.35), the above equation becomes:

$\begin{matrix}{{I.V.} = \frac{\sqrt{1 + {1.4\left( {R.V.{- 1}} \right)}} - 1}{0.35}} & \left( {{dl}/g} \right)\end{matrix}$

The viscosity of the polyester can be indicated either as intrinsicviscosity (IV) or as average molecular weight (number average: Mn). Forconverting an IV value, measured in phenol/dichloromethane=1:1, into theaverage molecular weight, the equationIV=k·Mn ^(a)is used, wherein k=2.1·10⁻⁴ and a=0.82.

This equation can be generally applied to published data, unless adifferent solvent mixture and the corresponding conversion factors areindicated.

According to a preferred embodiment of the present invention, thepolycondensate pellets flow essentially downwardly through the secondtreatment space, while a process gas flows through the second treatmentspace in countercurrent or alternatively in cross-current or inco-current. Polymer particle movement in the crystallizer is due tomechanical movement or preferably due to the stream of process gas.

The process gas can be, for example, air, steam or inert gases such asnitrogen or CO₂. The process gas can comprise a mixture of severalprocess gases. The process gas may comprise additives, which eitherreact with the polycondensate to be treated, have a swelling action onthe polycondensate, or deposit passively on the polycondensate pelletsto be treated.

The crystallizer according to the invention is used in form of afluidized bed. In other words, in the entire second treatment space, orat least in a partial section of the second treatment space, which isprovided by the crystallizer, the conditions of a fluidized bed areestablished.

A fluidized bed is a bulk of solid particles which by means of anupwardly flow of a fluid are brought into a fluidized state. Spouted bedand moving bed are embodiments of a fluidized bed, resulting fromdifferent fluid flow and respectively different intensity offluidization.

The second treatment space is surrounded by a housing. The horizontalcross section of the treatment space can have any desired shape, butpreferably is round or rectangular. The disposition of the treatmentspace is preferably essentially vertical, so the pellet material canflow downwardly through the apparatus. It is important here to achieve aproduct flux which is uniform. The second treatment space is laterallylimited by a casing. The casing wall can consist of cylindrical orconical segments or of a combination of conical and cylindricalsegments, whereby the gas speed distribution can be influenced via theheight of the apparatus. Widening in the ceiling region facilitates areduction in the gas speed to prevent the discharge of pellets.Narrowing in the ceiling region facilitates an increased gas speed,leading to greater swirling to prevent any sticky agglomerations.

One particular embodiment of the present invention provides an at leastapproximately rotationally symmetrical housing casing, which results infabrication advantages as well as advantages in respect of regularity ofproduct flux.

In the interior of the second treatment space displacers can be providedwhich do not have pellet material flow through it and accordingly reducethe size of the second treatment space. Displacers of this type can beused for example for routing of process gas, for adjusting the freecross-sectional area or for improving pellet flux.

Inside the second treatment space, partition walls can be provided,which divide the second treatment space into two or more chambers,wherein the chambers may be connected with each other by means ofpassage openings for the pellets.

Preferably, at least one feed opening empties into the ceiling region ofthe second treatment space and facilitates the importation into thesecond treatment space of the pellet material to be treated. The feedopening can be for example an opening in the housing, or the exit from apipe leading into the housing. The feed opening can be subdivided intotwo or more segments, which enables disbursement of the pellets in thetreatment space.

Preferably, at least one discharge opening empties into the lower partof the second treatment space and the treated pellet material can bedischarged there through from the treatment space. The discharge openingcan be for example an opening in the housing or the entry into a pipeleading out of the housing. Usually, the pellet material travels in thedirection of the discharge opening through a conical region. The angleof the outflow cone with the horizontal is preferably 50-80° when thepellet material is not fluidized or oscillated in the discharge cone and15-60°, especially 30-50°, when the pellet material is fluidized oroscillated in the discharge cone. Alternatively, the pellet material canalso be fed in the direction of the discharge opening via a mechanicaldischarge apparatus, for example a screw.

According to a preferred embodiment of the invention, a shut-off device,preferably a rotary air lock unit such as a cellular wheel lock, or agrooved roller unit, is located underneath the discharge opening and isused to control the efflux of pellet material out of the treatmentspace. The controlled variable here can be, for example, the fill heightof the pellet material in the treatment space or the weight of thepellets in the apparatus.

Said shut-off device, preferably rotary air lock unit, is provided witha housing and a rotor movably arranged therein, wherein the shut-offdevice, preferably rotary air lock unit, comprises a gap between therotor and the housing which is larger than the average pellet diameter.

Due to the fact that in a fluidized bed (moving bed or spouted bed) withsuch short residence time as defined according to the present inventionthere may be formed a portion of pellets with low crystallinity (whichare very soft and deformable), the problem arises that in a subsequentshut-off device, which regulates the discharge of pellets from thefluidized bed (and which thus is always completely filled with pellets),a portion of the soft pellets may become squashed. According to apreferred embodiment of the present invention, this problem is solved bya discharge through a shut-off device, preferably a rotary air lockunit, with greater range of tolerance. By providing a gap between rotorand housing which is larger than the average pellet diameter, squashingof pellets in the shut-off device, preferably rotary air lock unit, isreduced or completely prevented.

By using such a shut-off device, preferably rotary air lock unit, withgreat range of tolerance, however the problem can occur that thecrystallizer with the second treatment space at this discharge openingdoes no longer exhibit a reliable gas stop. For certain applications,this can be a serious drawback.

As a solution for this problem, according to a preferred embodiment ofthe present invention there is provided, downstream of the shut-offdevice, preferably rotary air lock unit, with great range of tolerancedescribed above, a second shut-off device, preferably rotary air lockunit, with a housing and a rotor movably provided therein, which has agap between rotor and housing which is smaller than the average pelletdiameter. This second shut-off device, preferably rotary air lock unit,due to its size and optionally due to its speed of revolution, cannot becompletely filled with pellets and thus cannot pinch or squash pellets.On the other hand, this second shut-off device, preferably rotary airlock unit, due to its dimensions is a reliable gas stop. The range oftolerance of the second shut-off device herewith preferably lies between0.05 and 1 mm.

The basal region of the treatment space preferably locates one or morethan one importing means for a process gas. The importing means has atleast one entry opening where through the process gas flows into thesecond treatment space.

The feed means for a process gas may comprise apparatuses, for exampleopen-base cones or series of ridge structures, or else lines or metalsheets with exit holes, as long as distribution of the process gas issufficiently uniform. In one particular embodiment, the treatment spaceis downwardly limited by an at least to some extent gaspermeable barriermeans, especially a perforated metal sheet with a multiplicity of entryopenings where through process gas can flow in some places at least, butpellets cannot. To this end, the openings are smaller than the diameterof the pellets. The permeable area is preferably between 1% and 30%.Preference is given to openings at between 20 and 90%, especiallybetween 30 and 80%, of the diameter of the pellets. The number, size andarrangement of the openings here can be uniform or non-uniform. Thedisposition of the barrier means is conical or horizontal.

Underneath the barrier means there can be a distributor space wherethrough process gas is conducted to the barrier means. At least oneopening for importing process gas empties into this distributor space.There can further be apparatuses for distributing the process gas, suchas baffle plates, valves or flaps, or else separate channels forindividual importation of process gas. Alternatively, the treatmentspace can be downwardly limited by a non-gaspermeable barrier means. Inthis case, the at least one importing means for a process gas can be anopening in the housing, the exit from one or more pipes leading into thehousing, or a single ridge structure or a series of ridge structures,these having either holes or open bases. Any displacer here can be usedfor gas importation.

In one particular embodiment of the present invention, there is, inaddition to the at least one feed means for process gas in the basalregion of the second treatment space, at least one further process gasimportation means which empties into the second treatment space, wherebya multi-stage heat supply as well as a multi-stage gas speed profile isachievable.

The ceiling region of the second treatment space preferably comprises atleast one removal means for the process gas. The removal means can befor example an opening in the housing or the entry into a pipe leadingout of the housing. The removal means in question can be situated in thecasing or ceiling of the treatment space. In an alternative embodimentof the invention, no such removal means is provided. In this case, theprocess gas leaves the second treatment space through the connectionline from the first to the second treatment space.

There can be devices within or underneath the removal means to permitthe passage of process gas but prevent the passage of pellets. This canbe effected for example with the aid of a curved or deflectedthrough-flow channel or with the aid of deflecting internals, forexample a zig-zag separator.

The pellet material is heated in the second treatment space by supply ofenergy from the outside, with the aid of the hot process gas. In thisrespect, the process gas is conducted into the second treatment space ata temperature T_(Gas) which is higher than the sum of the pellettemperature T_(GR) and the temperature increase T_(KR) which occurs dueto heat of crystallization released in the second treatment space, i.e.T_(Gas)>(T_(GR)+T_(KR)). The gas temperature T_(Gas) thus lies above theaverage pellet temperature which the polycondensate pellets would reachwithin the second treatment space without external heat supply, theresulting advantage being that the polycondensate pellets can be set toa constant and defined exit temperature. According to the presentinvention, it could be shown that for obtaining a homogeneouslycrystallized polycondensate the external heat supply has to be carriedout under consideration of the temperature increase T_(KR) which occursdue to heat of crystallization released in the second treatment space.

According to a preferred embodiment of the present invention, the ratioX of the mass flows of gas (m_(G)) and pellets (m_(P)) (X=m_(G)/m_(P))is adjusted in the second treatment space such that1≤(T_(Gas)−T_(KR)−T_(GR))*X≤80, preferably2<(T_(Gas)−T_(KR)−T_(GR))*X≤80, especially preferred4<(T_(Gas)−T_(KR)−T_(GR))*X≤80. By this, polycondensate with especiallypreferred characteristics of homogenous crystallization is obtained.

Due to the increase in crystallinity, heat of crystallization isreleased. Due to this heat of crystallization, there occurs atemperature increase T_(KR) which results from the amount of releasedheat divided by the specific heat capacity of the polycondensate.

Generally, the temperature increase T_(KR) in the second treatment spacewhich results from the released heat of crystallization lies in a rangefrom 5° C. to 40° C. In particular, in the case of polyethyleneterephthalate (PET) T_(KR) lies in a range from 10° C. to 30° C.

According to a preferred embodiment of the present invention, pelletmaterial is heated in the second treatment space (i.e. in thecrystallizer) to a temperature which is 30° C. or more below the melttemperature of the polycondensate. In the case of polyethyleneterephthalate (PET), the pellets are preferably heated in the secondtreatment space (i.e. in the crystallizer) to a temperature in the rangefrom 140 to 220° C., particularly preferred to 150° C. to 190° C.

According to a preferred embodiment of the present invention, theresidence time of the material is preferably in the range from 1 to 30crystallization half times. In the case of a rapidly crystallizingpolymer such as polyethylene terephthalate (PET) having a comonomercontent of less than 5%, the residence time is accordingly in the rangefrom 1 to 30 minutes, preferably 1 to 15 minutes and especiallypreferred 1 to 8 minutes. Slow-crystallizing polymers accordingly haveto be longer resident in the second treatment space until the desiredincrease in crystallinity has been attained.

According to a preferred embodiment of the present invention, thecrystallization in the crystallizer is effected with an inert gas asprocess gas, in contrast to conventional processes. Use of nitrogen ispreferred. In this respect, the oxygen content of the gas in the secondtreatment space shall be less than 1% by weight, preferably less than0.5% by weight and more preferably less than 0.1% by weight in order toreduce/avoid oxidative damage to the material under the conditions ofthe crystallization.

Since heated process gases are costly, the invention provides that theprocess gas used in the crystallizer is preferably routed at leastpartly in a circuit system where a small amount of exchange gas can beimported and a small amount of exchange gas can be removed. In thispreferred embodiment, there is a closed-loop circuit of pipework linesbetween the removal means and importation means for the process gas.

The circuit may contain further units, for example compression means(e.g. ventilators, blowers or compressors), heat exchangers (e.g.heaters), blocking devices (e.g. a valve or cock) or cleaning means(e.g. filters, cyclones, scrubbers or catalytic combustion means). Sincea large proportion of the compression energy passes as heat into the gasstream, the elevated process gas temperature can also be maintained withless additional heater performance. However, if the entry temperature ofthe polycondensate pellets changes, this can be compensated by means ofthe additional heater performance. According to the present invention,the closed-loop circuit system preferably contains additional unitsselected from the group consisting of a ventilator, a heat exchangersuch as a heater, a blocking device, a condenser and a combinationthereof.

According to a preferred embodiment of the present invention, thecrystallizer communicates with an inert gas tank, preferably via a feedline. Through the feed line, inert gas can be fed from the inert gastank into the circuit system of pipework lines or directly into thecrystallizer. Preferably, a metering unit, for example a control valve,is disposed in this feed line and can be used to control the importationof inert gas.

According to a preferred embodiment of the present invention, thepressure p2 in the second treatment space is set to be above thepressure p1 in the first treatment space. This prevents cooling mediumtransitioning from the first treatment space in the dryer into thesecond treatment space in the crystallizer. Owing to the higher pressurein the second treatment space, inert gas does transition from the secondtreatment space into the first treatment space. However, it hastranspired that according to a preferred embodiment of the presentinvention it is sufficient for the purposes of the present inventionwhen p1<p2<p1+100 mbar, preferably p1<p2<p1+50 mbar, i.e. the pressurein the second treatment space is less than 100 mbar, preferably evenless than 50 mbar, above the pressure in the first treatment space. Inother words, a slightly higher pressure in the second treatment spacerelative to the first treatment space is surprisingly sufficient. Underthese pressure conditions, only a small proportion of inert gas is lostby transitioning into the first treatment space. This small disadvantageis more than compensated by the low moisture level in the crystallizerand the associated advantages with regard to equipment design andprocess engineering (smaller amount of inert gas needed in thecrystallizer, inert gas needs minimal or no drying).

The apparatus of the present invention in one preferred embodimentcomprises sensors to determine the pressure in the first and secondtreatment spaces. Conventional pressure sensors can be used, accordingto the present invention. The sensors communicate with a control unitwhich can evaluate the data ascertained by the sensors. The control unitaccording to this embodiment of the present invention is usually acomputer having requisite components such as a central processing unit(CPU) and memories.

According to a preferred embodiment of the present invention, thecontrol unit preferably communicates with the metering unit in the feedline and polices the metering unit to control the amount of process gasfed to the circuit system of the crystallizer. If, for example, thecontrol unit uses the pressure values transmitted by the sensors toascertain a pressure increase in the first treatment space, the meteringunit is opened to let such an amount of process gas pass into thecircuit system that the desired higher pressure in the second treatmentspace becomes established. Similarly, in the event of a pressuredecrease in the first treatment space, a metering unit which is situatedin a separate let-off line and likewise communicates with the controlunit and is controlled thereby opens to let off process gas out of thecircuit system in order to prevent excess pressure in the secondtreatment space with an attendant excess transition of process gas fromthe second treatment space into the first. Alternatively, the desiredpressure difference between the first and second treatment spaces canalso be set by opening appropriate metering means to increase or reducethe air importation into the first treatment space and/or the airexportation from the first treatment space.

If the inert gas quantity, which is needed to adjust the overpressure inthe second treatment space desired according to a preferred embodimentof the present invention, is known or has been ascertained, according toa preferred embodiment of the present invention the apparatus can alsobe operated without control unit by passing the requisite quantity ofinert gas into the crystallizer. However, even in this case it is atleast advisable to police the pressure conditions in the first andsecond treatment spaces by means of pressure sensors. A furtheralternative embodiment provides no gas outlet from the second treatmentspace. In this case, the passing of inert gas into the second treatmentspace will inevitably lead to an increased pressure in the secondtreatment space relative to the first treatment space. This, at acertain minimum pressure gradient, will lead to the inert gastransitioning from the second into the first treatment space andthereby, in accordance with a preferred embodiment of the presentinvention, likewise prevents the ingress of water into the secondtreatment space.

According to a preferred embodiment of the present invention, any energyloss in the crystallizer due to possible evaporation of cooling mediumfrom the pellet material is kept very small.

According to this preferred embodiment of the present invention,therefore, a small amount of process gas is needed to efficiently heatthe pellet material to a temperature which corresponds to a temperatureto be applied in any subsequent aftertreatment stage. According to apreferred embodiment of the present invention, therefore, furtherheating of the pellet material in an aftertreatment stage subsequent tothe crystallization is only required to a minimal extent, if at all.

On exit from the second treatment space the pellet material preferablyhas a degree of crystallinity which is greater than 35% of the degree ofcrystallinity which could be maximally obtained at this crystallizationtemperature. In particular, on exit from the second treatment space thepellet material preferably has a degree of crystallinity which isbetween 40% and 70% of the degree of crystallinity which could bemaximally obtained at this crystallization temperature. In the case ofpolyethylene terephthalate (PET), on exit from the second treatmentspace the pellet material has a degree of crystallinity between 20% and50%, especially between 30% and 40%.

According to the present invention, the material obtained after thecrystallization can be subjected to a further thermal treatment step,which is preferably selected from the group of a devolatilizing step,preferably a de-aldehydization step, and a solid-state polymerization(SSP). This thermal treatment step is carried out in a third treatmentspace, which is preferably situated in a separate reactor.

Both the devolatilization, preferably de-aldehydization, and the SSPreaction are known to a person skilled in the art and need not befurther elucidated herein. According to the present invention, thisthermal treatment is preferably carried out in a third treatment spacein a gas phase of inert gas, and causes volatile components to evaporatefrom the polymer during the treatment and pass into the gas phase.Nitrogen is preferably used as the inert gas. According to the presentinvention, the oxygen content of the gas in the third treatment spaceshall be less than 0.1% by weight, preferably less than 0.05% by weightand more preferably less than 0.02% by weight in order to reduce/avoidoxidative damage to the material under the conditions of thecrystallization.

Owing to the high temperature of the polymer entering the thirdtreatment space, there is no need to pass very hot process gas into thethird treatment space. Owing to evolving heat of crystallization, atemperature increase of 3 to 15° C., preferably of 5 to 15° C. and morepreferably of 7 to 15° C. is generally observed in the third treatmentspace of the invention in the case of polyethylene terephthalate (PET).In the present invention, therefore, process gas passed into the thirdtreatment space can in certain cases have a temperature below thetemperature of the polymer particles in the third treatment space. Thepresent invention accordingly ensures that a subsequent thermaltreatment step can be carried out in an energy-efficient manner.

The flow of the process gas in the third treatment space is preferablyin counter current to the stream of polymer particles. The same reactorscan in principle be used in the thermal treatment as were describedabove for the crystallization, or other reactors conventionally used forthis purpose. A conventional shaft reactor may be mentioned as anexample.

The crystallized particles of polymer are transferred out of thecrystallizer into the third treatment space in a known manner. Forexample, the particles can be pneumatically conveyed from thecrystallizer into the third treatment space. Depending on the speed ofconveyance, the polymer particles in the case of polyethyleneterephthalate (PET) can cool down by between 2 and 15° C., preferablybetween 3 and 15° C. and more preferably between 5 and 15° C. Ifnecessary, the particles can be additionally heated in a known mannerbefore entry into the third treatment space.

According to the present invention, the de-aldehydization of, forexample, polyethylene terephthalate can take place at a temperature of140 to 200° C. in the third treatment space. A temperature increase inthe third treatment space of 3 to 15° C. is observed in the course ofthis de-aldehydization, for example.

According to the present invention, the SSP reaction of, for example,polyethylene terephthalate can take place at a temperature of 180 to240° C., preferably 180 to 225° C. in the third treatment space. Atemperature increase in the third treatment space of 3 to 15° C. isobserved in the course of this SSP reaction, for example.

The present invention will now be further elucidated using anon-limiting drawing, where

FIG. 1 shows a schematic depiction of a preferred embodiment of anapparatus according to the present invention.

The apparatus according to FIG. 1 has a reactor 1 for producing apolymer melt. This can be a reactor in which a melt polymerization iscarried out to thus produce a prepolymer from the monomers.Alternatively, reactor 1 can also be an apparatus for melting a solidproduct, for example a prepolymer. Reactor 1 can in this case be anextruder for example.

The molten material is transferred into a pelletization apparatus 2. Inthe pelletization apparatus 2, a pellet material is produced from themolten material in a known manner. This may involve, for example, anunderwater pelletizer (as shown in FIG. 1) or an underwater strandpelletizer. Pelletization takes place under water in this case. Thepellets obtained are concurrently cooled down in pelletizer 2. As noted,however, cooling must not be so severe as to cool the pellets to belowtheir crystallization temperature range. This can be achieved via theuse of heated water having a temperature above 50° C., but at least 10°C. below it pressure-dependent boiling point, in particular having atemperature below the Tg of the polycondensate, especially having atemperature from 60 to 80° C. The pellet material should in the case ofpolyethylene terephthalate (PET) be cooled to a temperature in the rangefrom 110 to 180° C., preferably 115 to 160° C. and especially 120 to150° C.

The pellet material is transferred via a connection line 3 directly intothe unit for drying the pellet material (drying unit) 4. To stop thepellet material from cooling down too much, this pellet material shouldbe conducted as quickly as possible out of the pelletization apparatus 2and through the connection line 3. Preferably, the flow velocity inconnection line 3 can be increased by passing a gas stream (preferablyair) into it.

The pellet material is separated from the liquid cooling medium (water)and dried in the unit for drying the pellet material (drying unit) 4.The cooling medium separated off is conducted via a pipework line 9 aback into the stock reservoir vessel (tank) 9 b for the cooling medium.The stock reservoir vessel 9 b has an inlet 9 e for importation ofcooling medium. From the stock reservoir vessel 9 b, the cooling mediumis transferred into the pelletization apparatus 2 by means of acirculation apparatus (pump) 9 c. In the course of the transfer, thecooling medium preferably traverses a heat exchanger 9 d. In the heatexchanger 9 d, the cooling medium can be heated or cooled down, asrequired. Especially cooling medium returned from the drying unit 4 canhave an excessive temperature because of the contact with hot pelletmaterial and has to be cooled before entry into the pelletizationapparatus 2.

The fresh cooling medium, which is added via the inlet 9 e, can containa basic medium or a pH buffer medium. In particular, the use of waterwith a neutralising or buffer effect which is adjusted within a narrowrange is provided herein. Alternatively, the addition of a basic mediumor of a pH buffer medium can also be effected directly into the coolingcircular system, e.g. into the storage container 9 b.

The drying of the pellet material in the first treatment space of unit 4is effected at a temperature of 100 to 200° C., preferably 120 to 160°C., by means of air, or a gas atmosphere comprising essentially air, aswell as a mechanical drying apparatus. In the apparatus of FIG. 1, theair is conducted into drying unit 4 via an air inlet 10 a. The air inlet10 a can be situated in the housing of drying unit 4 or in theconnection line 5 or in both locations. Optionally, an aspirating filter(not illustrated) can be disposed in the air inlet line 10 a. The airleaves the drying unit 4 through the air outlet 10 b. A ventilator 10 cfor circulating the air through the drying unit 4 is disposed in the airoutlet line 10 b in the apparatus of FIG. 1. However, the ventilatorcould alternatively also be disposed in the air inlet line 10 a.Furthermore, air inlet 10 a and air outlet 10 b can be connected to eachother to form a circuit system. A condenser would then have to beprovided in this circuit system.

The pellet material is transferred from the dryer 4 via a connectionline 5 directly into a crystallizer 6. To avoid pellet adherence andagglomeration, connection line 5 is not equipped with shut-off devices.The pellet material can transition unimpeded from dryer 4 intocrystallizer 6.

In crystallizer 6, the essentially amorphous pellet material is at leastpartially crystallized. Within crystallizer 6, the pellets are keptmoving by a gas stream passing through crystallizer 6. Withincrystallizer 6, the conditions are those of a fluidized bed.Additionally, an apparatus for mechanical movement of the particlescould also be provided in crystallizer 6.

The pellets are crystallized by external heat supply, wherein forexternal heat supply a process gas in counter current to the pellet flowis led through the second treatment space, said process gas having atemperature T_(Gas) which is higher than the pellet temperature T_(GR)and the temperature increase T_(KR) in the second treatment space whichoccurs due to released heat of crystallization, i.e.T_(Gas)>(T_(GR)+T_(KR)). In the case of polyethylene terephthalate (PET)crystallization occurs at a temperature of 140 to 220° C., wherein attemperatures above 180° C. there is preferably used an inert gas,essentially nitrogen, as process gas. The crystallized pellet materialdeparts the crystallizer via a discharge apparatus 7, for example ashut-off device such as a cellular wheel lock. Said cellular wheel lockis provided with a housing and a rotor movably arranged therein, whichhas a gap between rotor and housing which is larger than the averagepellet diameter.

Alternatively, downstream of the cellular wheel lock there can beprovided a second rotary air lock unit (such as a cellular wheel lock)with a housing and a rotor movably arranged therein, which has a gapbetween rotor and housing which is smaller than the average pelletdiameter.

The pellets can be subjected to a subsequent thermal treatment such as ade-aldehydization or SSP reaction. Alternatively, the pellets can alsobe sent into a cooling step.

The process gas used in crystallizer 6 is conducted through aclosed-loop circuit system of pipework lines 8 a. The process gas enterscrystallizer 6 through an inlet 6 a and departs crystallizer 6 throughthe outlet 6 b. The circuit system for the process gas contains aventilator 8 b for circulating the gas. A heat exchanger 8 c is providedupstream of inlet 6 a to bring the gas to the desired temperature beforeentry into crystallizer 6. Preferably, the gas is heated in heatexchanger 8 c.

The circuit system of FIG. 1 further comprises a blocking device 8 d (avalve, for example) to conduct a portion of the gas in a secondarycircuit.

The circuit system of FIG. 1 further contains a condenser 8 e to removecooling medium from the gas, absorbed by the gas during passage throughcrystallizer 6. Since, according to the present invention, littlecooling medium transitions into crystallizer 6, the dimensions ofcondenser 8 e can be comparatively small.

The apparatus of FIG. 1 has a feed line 11 for fresh process gas, viawhich process gas can be fed into the above-described circuit systemfrom an inert gas tank (not illustrated) or be let off out of thecircuit system.

The present apparatus of the present invention is very useful forcontinuous pelletization and crystallization of a polymer, especially apolycondensate, preferably a polyester such as polyethyleneterephthalate.

The invention claimed is:
 1. Process for continuous production of partlycrystalline polycondensate pellet material, comprising the steps of a)forming a polycondensate melt into pellet material by adding a liquidcooling medium, which has a temperature below the glass transitiontemperature (Tg) of the polycondensate, and cooling to an average pellettemperature within the range of temperature of crystallization of thepolycondensate, wherein cooling takes place before or during or afterforming to pellets; b) separating the liquid cooling medium from thepellet material in a first treatment space, wherein the pellets afterexit from the first treatment space exhibit a temperature T_(GR), c)crystallizing the pellet material in a second treatment space, whereinin the second treatment space fluidized bed conditions exist, and in thesecond treatment space the pellets are heated by supply of energy fromthe exterior by means of a process gas, wherein the process gas has atemperature T_(Gas), which is higher than the sum of the pellettemperature T_(GR) and the temperature increase T_(KR) which occurs dueto heat of crystallization released in the second treatment space, i.e.T_(Gas)>(T_(GR)+T_(KR)) and wherein the pellets at the exit from thesecond treatment space have an average temperature T_(PH), which is 5 to70° C. higher than the sum of the temperature of the pellets T_(GR) andthe temperature increase T_(KR) which occurs due to heat ofcrystallization released in the second treatment space, i.e.(T_(GR)+T_(KR)70° C.)≥T_(PH)≥(T_(GR)+T_(KR)+5° C.).
 2. Process accordingto claim 1, wherein the polycondensate is selected from the groupconsisting of polyesters and polyamides.
 3. Process according to claim2, wherein the polycondensate is a polyethylene terephthalatehomopolymer of copolymer.
 4. Process according to claim 3, whereindrying of the pellets occurs in the first treatment space, and whereinafter exit from the first treatment space the pellets have a temperatureT_(GR) in the range from 100 to 180° C.
 5. Process according to claim 1,wherein the cooling medium is water.
 6. Process according to claim 5,wherein the water has a temperature in the range from 50° C. to 80° C.7. Process according to claim 1, wherein the ratio X of the mass flowsof gas (m_(G)) and pellets (m_(P)) (X=m_(G)/m_(P)) is adjusted in thesecond treatment space such that 1≤(T_(Gas)−T_(KR)−T_(GR))*X≤80. 8.Process according to claim 1, wherein the residence time of the pelletsin the second treatment space is in the range from 1 to 15 minutes.